LAWSONITE BLUESCHISTS AND LAWSONITE ECLOGITES AS PROXIES FOR PALEO-SUBDUCTION ZONE PROCESSES: A REVIEW

Lawsonite-bearing metamorphic/metasomatic rocks form under high-pressure–ultrahigh-pressure (HP–UHP) and low-temperature (LT) conditions common in Pacific-type subduction zones. The P-T stability fields of lawsonite blueschists and lawsonite eclogites represent subfacies of the blueschist- and eclogite-facies, respectively. Although the lawsonite–epidote transition boundary has a positive dP/dT-slope, the blueschist-to-eclogite transformation within the lawsonite stability field is gradational (multivariant) and cannot be defined by a specific discontinuous (univariant) reaction. The oldest occurrences of lawsonite-bearing blueschists are late Neoproterozoic, suggesting that subduction-zone thermal structures evolved toward the necessary LT conditions for lawsonite formation only by the end of the Proterozoic. A clear difference in frequency between Phanerozoic lawsonite and epidote blueschists does not exist, but our new compilation found a global lawsonite hiatus in the Permian that is a robust indication of relatively warm subduction zone thermal regimes. Lawsonite eclogites have been confirmed from more than eighteen localities; they are classified as L- (lawsonite), E- (epidote), and U- (coesite) type (Tsujimori et al., 2006; Wei and Clarke, 2011). Complete preservation of L-type lawsonite eclogites attending their return to the surface is uncommon. Rare evidence of progressive eclogitization within the lawsonite stability field is preserved in some zoned garnets, as growth isolates a significant volume of precursor phases and textures during incipient eclogitization. Brittle fracturing and fluid infiltration during prograde eclogite-facies metamorphism are common phenomena. Certain lawsonite-bearing metasomatic rocks record multiple fluid-infiltration events. Significant cooling and continuous H₂O supply from the devolatilizing oceanic plate to exhuming HP serpentinite mélange prevent dehydration breakdown of the lawsonite during decompression. The subduction records of lawsonite blueschists and eclogites agree with numerical modeling of subduction zones.